[CSM] sound_play & sound_stop support + client_lua_api doc (#5096)

* squashed: CSM: Implement register_globalstep
  * Re-use fatal error mechanism from server to disconnect client on CSM error
  * Little client functions cleanups

* squashed: CSM: add core.after function
  * core.after is shared code between client & server
  * ModApiUtil get_us_time feature enabled for client

builtin/common/after.lua

@@ -0,0 +1,43 @@
+local jobs = {}
+local time = 0.0
+local last = core.get_us_time() / 1000000
+
+core.register_globalstep(function(dtime)
+	local new = core.get_us_time() / 1000000
+	if new > last then
+		time = time + (new - last)
+	else
+		-- Overflow, we may lose a little bit of time here but
+		-- only 1 tick max, potentially running timers slightly
+		-- too early.
+		time = time + new
+	end
+	last = new
+
+	if #jobs < 1 then
+		return
+	end
+
+	-- Iterate backwards so that we miss any new timers added by
+	-- a timer callback, and so that we don't skip the next timer
+	-- in the list if we remove one.
+	for i = #jobs, 1, -1 do
+		local job = jobs[i]
+		if time >= job.expire then
+			core.set_last_run_mod(job.mod_origin)
+			job.func(unpack(job.arg))
+			table.remove(jobs, i)
+		end
+	end
+end)
+
+function core.after(after, func, ...)
+	assert(tonumber(after) and type(func) == "function",
+		"Invalid core.after invocation")
+	jobs[#jobs + 1] = {
+		func = func,
+		expire = time + after,
+		arg = {...},
+		mod_origin = core.get_last_run_mod()
+	}
+end